Zeolites are a class of important materials used in the chemical industry for processes such as gas stream purification and hydrocarbon conversion processes. Zeolites are porous solids having interconnected pores of different sizes. Zeolites typically have a one-, two- or three-dimensional crystalline pore structure that selectively adsorb molecules that can enter the pores and exclude those molecules that are too large. The pore size, pore shape, interstitial spacing or channels, composition, crystal morphology and structure are a few characteristics of zeolites that determine their use in various adsorption and hydrocarbon conversion processes.
Aluminosilicate zeolite catalysts are widely used in petrochemical and oil refining processes and in fine chemical synthesis because their strong acid sites within uniform micropores give both high activities and shape selectivities. However, their applications are limited due to the small aperture size (<2 nm) of the micropores. Increasing the accessibility of bulky molecules to and from catalytic sites can widen the range of reactions catalyzed by zeolites.
In recent years, there has been considerable effort in making microporous zeolite materials with higher surface area and, therefore, shorter path lengths to and from the catalytically active sites. Such methods include the synthesis of zeolite nanocrystals, the delamination of layered zeolites, and the introduction of mesopores into microporous material by various templating strategies or demetallation processes. Zeolites nanocrystals have been prepared only in a limited number of structures and often in low yield. Conventional delamination methods use expensive organic surfactants to effect delamination and require harsh pH conditions which can lead to partial destruction of zeolites. Concerns about the scalability, stability and regenerability of combined mesoporous/microporous materials have limited their industrial applicability. Accordingly, there is a continued need for new methods for making zeolites having higher external surface area.
Zeolite SSZ-70 is a known crystalline material and is useful in many processes, including various catalytic reactions. See, e.g., U.S. Pat. Nos. 7,083,767; 7,084,304; 7,084,305; 7,108,843; and 7,550,073. Although the crystal structure of SSZ-70 remains unknown, SSZ-70 appears to be a layered material having structural features similar to SSZ-25/MCM-22 (MWW). MWW-based catalysts are employed in various commercial processes.
U.S. Pat. No. 7,108,843 discloses the preparation of SSZ-70 in fluoride-containing media using boron and a N,N′-diisopropyl imidazolium cation structure directing agent. Post-synthetic replacement of the boron in the borosilicate SSZ-70 (B-SSZ-70) framework with aluminum was required for catalytic activity. Modest catalytic activity in acid-catalyzed hydrocarbon conversion reactions was reported, possibly due to incomplete Al-exchange.
U.S. Patent Application Publication No. 2010/0260665 discloses the direct synthesis of pure-silica, borosilicate and aluminosilicate SSZ-70 zeolites in either fluoride- or hydroxide-containing media using a variety of N,N′-disubstituted imidazolium cation structure directing agents. Aluminosilicate SSZ-70 (Al-SSZ-70) exhibited a cracking rate deactivation similar to SSZ-25 (MWW) suggesting the presence of a similar cavity, but the absence of an increasing Constraint Index value as Al-SSZ-70 deactivated suggests a second pore system distinct to the sinusoidal 10-membered ring pore system found in MWW zeolites.
Conventional forms of zeolite SSZ-70 tend to have an external surface area of less than 50 m2/g. Accordingly, there is a continued need for new methods for making SSZ-70, particularly forms of this material having higher external surface area.